The major pumping chambers in the human heart are the left and right ventricles. The simultaneous physical contraction of the myocardial tissue in these chambers expels blood into the aorta and the pulmonary artery. Blood enters the ventricles from smaller antechambers called the left and right atria which contract about 100 milliseconds (ms) before the ventricles. This interval is known as the atrioventricular (AV) delay. The physical contractions of the muscle tissue result from the depolarization of such tissue, which depolarization is induced by a wave of spontaneous electrical excitation which begins in the right atrium, spreads to the left atrium and then enters the AV node which delays its passage to the ventricles via the so-called bundle of His. The frequency of the waves of excitation is normally regulated metabolically by the sinus node. The atrial rate is thus referred to as the sinus rate or sinus rhythm of the heart.
Electrical signals corresponding to the depolarization of the myocardial muscle tissue appear in the patient's electrocardiogram. A brief low amplitude signal known as the P-wave accompanies atrial depolarization normally followed by a much larger amplitude signal, known as the QRS complex, with a predominant R-wave signifying ventricular depolarization. Repolarization prior to the next contraction is marked by a broad waveform in the electrocardiogram known as the T-wave.
A typical implanted cardiac pacer (or pacemaker) operates by supplying missing stimulation pulses through an electrode on a pacing lead in contact with the atrial or ventricular muscle tissue. The electrical stimulus independently initiates depolarization of the myocardial (atrial or ventricular) tissue resulting in the desired contraction. The P-wave or R-wave can be sensed through the same lead (i.e., the pacing lead) and used as a timing signal to synchronize or inhibit stimulation pulses in relation to spontaneous (natural or intrinsic) cardiac activity. The sensed P-wave or R-wave signals are referred to as an atrial electrogram or ventricular electrogram, respectively.
Note that the term electrogram lead is used herein to refer to the lead that transmits the sensed electrogram signal from the heart, and the term pacing lead is used to refer to the lead that transmits the stimulation pulse to the heart. As mentioned above, however, these "leads" are generally combined (i.e., the sensed electrogram signal is transmitted from the heart by the same lead that transmits the stimulation pulse to the heart). The separate terms "electrogram lead" and "pacing lead" are used herein merely to indicate that the electrogram signal and the stimulation pulse could be transmitted using separate leads.
Every modern-day implantable pacemaker includes a sensing circuit, whether the activity of one or both chambers of the heart are sensed. A cardiac event is sensed when an amplified electrogram signal exceeds a threshold value. If the sensitivity level is too low (i.e., the gain is too low), then some cardiac events will not be sensed because even peak signals may not exceed the threshold level. If the sensitivity level is too high, on the other hand, the high gain of the amplifier may cause noise or T-wave signals to be sensed, giving rise to erroneous sensing of cardiac events. Pacemakers provided with communications telemetry (e.g., noninvasive programming capabilities) advantageously allow the physician to set the sensitivity level.
There are at least two disadvantages to having the physician set the sensitivity level. First, adjusting the sensitivity level is one more thing that the physician must remember to do, and it would be advantageous to relieve him or her of that task if it is possible to do so. Second, and more important, the physician generally sees the patient only occasionally, and weeks or months may go by without the sensitivity level being changed. Problematically, the sensitivity level that will accurately detect cardiac events at a given threshold level for a patient does not stay static; both R-wave amplitude and frequency content can vary considerably within a given patient. Changes in the sensitivity level are needed to accommodate for physical and mental stress. In addition, the sensitivity level needs to change as myocardial tissue (heart muscle tissue) undergoes scarring or other physical responses to the implanted electrogram lead(s). Other changes in the myocardium-electrogram lead interface, e.g., shifting of the position of the electrogram lead, may also cause changes in the proper sensitivity level.
Unfortunately, these changes occur over a period of days and, in some cases, even hours or minutes. Because the physician generally sees the patient only every few weeks or months, the pacemaker sensing circuits can erroneously detect, or not detect, cardiac events over large periods of time. This erroneous detection/nondetection can cause under-pacing or over-pacing of the heart. Unfortunately for the patient, such changes may potentially leave him or her in a worse condition than he or she was in before the pacemaker was implanted. At best, the pacemaker is not able to operate efficiently--either by unnecessarily pacing and thereby draining the battery and risking pacemaker-induced tachycardias; or by not pacing as often as is needed by the patient. Thus, what is needed is a way to adjust the sensitivity level of a cardiac event detector in response to changing conditions in an electrogram signal over a short period of time.
One way of adjusting the sensitivity level of a cardiac event detector is discussed in U.S. Pat. No. 4,708,144 issued to Hamilton et al. The Hamilton et al. patent shows the use of an attenuator to attenuate an amplified signal before such signal is digitized and rectified. After the signal is digitized and rectified, the signal is connected to a digital comparator. The digital comparator compares each digitized and rectified sample to a threshold value. If the digitized and rectified sample exceeds the threshold value, a cardiac event is detected. In response to the detected cardiac event, a pacemaker control circuit takes appropriate action.
Each digitized and rectified sample is also presented to a peak detector which stores the maximum, or largest, digitized and rectified sample. The stored maximum sample is coupled to the pacemaker control circuit. After each cardiac event is sensed, the pacemaker control circuit averages the stored maximum with any of the previously occurring maximums yielding an average peak value. This average peak value is used to determine whether or not the attenuator should be adjusted to increase or decrease the attenuation provided by the attenuator. Specifically, if the average peak value increases, the attenuation is increased; and if the average peak value decreases, the attenuation is decreased, thereby adjusting the amplitude of the signal before it is digitized and rectified.
Disadvantageously, even though the Hamilton et al. circuit provides one technique for dynamically adjusting the sensitivity level of a cardiac event detector, by attenuating the cardiac signals after the amplification stage, it suffers from potentially clipping the input signal before even reaching the attenuation stage or the peak detecting stage. Furthermore, it completely lacks the ability to eliminate the sensing of high amplitude T-waves, which could cause the peak detector to erroneously detect the peak T-wave. Furthermore, if the amplitude of the T-wave is too high, or if the gain of the amplifier is so high that the R-wave is clipped and the T-wave is, by comparison, similar in amplitude to the clipped R-wave, it can result in "double sensing". Double sensing, when it occurs, then falsely indicates that a tachycardia is present. Thus, Hamilton et al. is not suitable for important cardiac monitoring functions beyond merely sensing a cardiac event. What is needed is a system which adjusts the gain at the pre-amplification stage for an optimum signal (i.e., without clipping the input signal) and then reliably eliminate sensing of the T-wave.
In addition to the detection of cardiac events, it is desirable, in the treatment of certain heart ailments, or for the detection of such ailments, to continuously monitor the patient over a certain period of time in order to determine the effectiveness of the treatment being administered by a pacemaker, under different conditions of stress or varying conditions of the heart. If the sensing circuit detects that the pacemaker is administering a less than ideal, or optimum, treatment, the treatment can be adjusted (e.g., by increasing or decreasing the rate at which pacing pulses are delivered, by decreasing the threshold level of the threshold detector, or by increasing the amplitude or duration of the pacing pulse).
Unfortunately, events that would indicate that the pacemaker may be providing less than the optimum treatment may occur only infrequently. Thus, a physician may not detect such abnormal events during a weekly, biweekly or monthly examination, which may last only a few minutes and may not be able to adjust the pacemaker accordingly. In an effort to solve this problem, data acquisition systems have been developed that record electrogram signals over a predetermined period of time, e.g., on the order of days. The electrogram signals may then be analyzed by a physician or, in more advanced system, by a microcontroller in the pacemaker in accordance with a control program that is designed to react to various conditions that are manifested by the electrogram signals. Such data acquisition systems advantageously allow detailed analysis of the electrogram signal over long periods of time thereby facilitating the detection and accommodation of infrequent heart abnormalities or the early detection of slowly developing heart ailments. Such long-term monitoring, particularly where implemented in advanced programmed systems, makes possible the purposeful and possibly automatic treatment of heart abnormalities long before the actual failure of a pacemaker to properly service the heart. In addition, with such automated systems, therapies, such as antitachycardia pacing and defibrillation, can be performed on the heart by pacing systems or dedicated defibrillators that would otherwise not be able to be performed as quickly or automatically.
Unfortunately, implanted data acquisition systems have heretofore only been operable over a limited sample of the electrogram signal. This is because such systems store the electrogram signal in a memory. The memory is of a limited size, and when the memory is full, either part of the previously recorded electrogram signal must be discarded to make room for new electrogram signal to be recorded, or the data acquisition system must stop recording. In an effort to solve this problem, various high capacity means of storing electrogram signals have been developed such as magnetic tape recording systems. For example, U.S. Pat. No. 4,250,888 issued to Grosskopf, suggests that when the memory is full, a warning message be given that alerts the patient to the need to contact the physician or to activate a tape recording system at home.
Disadvantageously, the Grosskopf approach may require that the patient report to a potentially inconvenient location, (i.e., the physician's office or the patient's home where the tape recording system is located). Such inconvenience may encourage the patient to ignore the warning message. In addition, the warning message can be intrusive and embarrassing. Furthermore, such warning systems are not used with implantable pacemakers for at least two reasons. First, implantable pacemakers are implanted within the body and, as such, any warning means are neither visible nor readily heard. Second, implantable pacemakers must be compact and use little power. Generally, the warning message is generated by a speaker or light source and thus draws a significant current. It is thus apparent that what is needed is an implantable cardiac event detection system that is not limited to operating on a small sample of the cardiac signal over a limited period of time, and that does not require the use of inconvenient and impractical storage devices such as tape recording systems.
Some systems have been developed that store only anomalous portions of the electrogram signal. See, e.g., Grosskopf. However, even these systems have a limited capacity and when a sufficient number of anomalous portions are stored, some data loss occurs. This data loss occurs when the memory is full and either the new signal must be discarded or the previously stored signal must be discarded. Problematically, the portion of the electrogram signal that is discarded may be the portion of signal that is needed for an accurate evaluation of the patient's heart condition. Thus, what is needed is an implantable cardiac event detection system that is not limited by the use of a finite capacity memory for storing the electrogram signal, but that provides information sufficient for programmed evaluation in a microcontroller and, if needed, automatic adjustment of a pacemaker or activation of a defibrillator in response to such evaluation.
Another problem faced by the designers of automated cardiac pacing and/or defibrillation systems is the need for analysis of the electrogram signal. One approach to accurately analyzing the electrogram signal requires that the stored electrogram signal be subjected to complex digital filtering algorithms and statistical analysis. See e.g., U.S. Pat. No. 4,422,459 issued to Simson. In order to generate the digitally filtered and statistically analyzed signals in Simson, a large computer system is employed. Such computer system is immobile and inconveniently located at, e.g., the physician's office, thus making implantation impossible. Disadvantageously, such algorithms and analysis require that many hundreds of mathematical operations be performed before an accurate conclusion as to whether the cardiac pacer and/or defibrillator are performing optimally can be obtained and, thus, before needed adjustment of the therapies provided by the cardiac pacer and/or defibrillator can be made. Problematically, this requires not only the use of a memory to store the incoming electrogram signal while the mathematical operations are being completed, the disadvantages of which are discussed above, but requires that many complicated computational steps be traversed by the microcontroller. Such complicated computational steps are highly power-consuming--which would require more frequent replacement of the battery that powers the implantable cardiac pacer and/or defibrillator--and thus, disadvantageous in implantable cardiac pacing applications.
Another approach to accurately analyzing the electrogram signal has been to allow the physician to analyze the electrogram signal stored in a memory using conventional electrogram analysis techniques. Disadvantageously, in order to obtain the stored electrogram signal, the physician must download the stored electrogram signal via a telemetry circuit in the cardiac pacer system and/or defibrillator system. Hence, because a memory is used, the problems discussed above are also present in this approach. A further disadvantage of this approach is that no automated adjustment of the therapies provided by the cardiac pacer and/or defibrillator can be made because such adjustment must wait until the patient has traveled to the physician's office and until the physician has completed his or her analysis. Thus, what is needed is an implantable cardiac data acquisition and analysis system that does not require the use of complicated and highly power-consuming mathematical computations.